The present invention generally relates to power electronic switching circuits and in particular to inverter modules employing two or more controlled switches.
Efficiency is becoming increasingly important in the field of power electronics and in many applications, such as inverter modules for the solar market, the efficiency optimi-zation turned out to represent a major design goal. Photovoltaic solar panels com-monly use PWM, pulse width modulation, inverters to convert DC power generated by the solar cell into AC power which can be fed into the power grid. Typical further appli-cations of these inverters include their use in uninterruptable power supplies, UPS, fuel cells and wind turbines. Further, PWM inverters may be used to provide compensation for reactive loads, harmonic cancellation of supply grids, or as variable speed drives for induction motors. The most commonly used inverters are one phase and three phase transformerless inverters.
The most common switching elements used in inverter designs are field effect transis-tors, FET, such as metal oxide semiconductor field effect transistors, MOSFET, bipolar transistors, such as insulated gate bipolar transistors, IGBT, bipolar junction transis-tors, BJT, and gate turn-off thyristor, GTO. Traditionally, MOSFETs have been used for low DC voltage or low power inverter designs. IGBTs have been used in medium to high power or high voltage inverter designs. GTOs have been used in very high power inverter designs.
In order to obtain low losses in an inverter, it is desirable to use transistors that have low switching losses and to use antiparallel/free-wheeling diodes across each transis-tor with good recovery behavior. MOSFETs are generally known to have very good switching performance but the internal antiparallel body diode exhibits poor recovery behavior. This diode can conduct current even if another current path is available, such as a parallel connected free-wheeling diode. When a MOSFET switch turns off, the current can transfer from the MOSFET channel into the parasitic body diode. When the control MOSFET turns on, the recovery charge stored in the body diode during conduction is swept out. Abrupt reverse recovery of the body diode can cause higher switching losses and high frequency ringing, which places higher stresses on the com-ponents and can couple to the output and can cause noise and electromagnetic inter-ference, EMI, associated problems. To compensate, inverter designs using MOSFETs have traditionally required the addition of both series and free-wheeling ultra fast di-odes. The addition of these diodes significantly increases the cost of the inverter de-sign and adds conduction losses. For these reasons IGBTs have been a more practi-cal choice for inverters operating above 100 to 200 VDC. IGBTs typically have poorer switching performance than MOSFETs, but require the addition of fewer diodes to pro-vide rapid recovery behavior, since the internal series diode present in IGBTs allows the designer to add a single diode to the free-wheeling path. The use of IGBTs can reduce the cost of an inverter design but may lower the inverter efficiency at higher frequencies.
Furthermore, when developing highly efficient inverter topologies which can handle reactive power, the intrinsic diode of the MOSFET will cause high reverse recovery losses when reactive power has to be managed. FIG. 12 shows an inverter circuit be-ing formed only by IGBTs which, however, is not fast enough for most novel solar ap-plications.
In FIG. 13 a combined IGBT and MOSFET inverter architecture is shown. This com-bined circuit, however, has the disadvantage that in the case of reactive power the intrinsic body diode causes losses which deteriorate the efficiency to an unacceptable extent.
Consequently, for future solar inverters a further improvement of efficiency and the ability to handle reactive power is needed.
In particular, the arrangement shown in FIG. 13 can only be used when dealing with a reactive load, if a fast recovery epitaxial diode field effect transistor, a so-called FRED-FET, is used. However, these components usually have a higher Rds-on and therefore the losses are significant. Another disadvantage is that their poor reverse recovery properties lead to an unsatisfactory performance in power factor compensation and bi-directional usage.
Accordingly, there is a need to provide an inverter power module that effectively con-trols and minimizes the effect of the body diode of MOSFETs and specifically, there is a need to provide means to reduce the adverse effects of EMI and the power loss due to the parasitic body diode.